In $,$ the U$.$S$.$ Army had contracts with civilian mental health care providers. In response to a significant increase in the cases of post$-$traumatic stress and other behavioral health issues, the army began contracting more providers at the rate of per year. Write a linear function describing the number of civilian mental health care providers. Then use your function to estimate the number of providers in $,$ and estimate how long it would have taken the number of providers to double from $.$